1. Field of the Invention
The present invention relates to charge carrier lifetime measurement of product semiconductor wafers.
2. Description of Related Art
It is well known in the art of semiconductor wafer processing that defects and impurities in the semiconducting material of the semiconductor wafer can affect the lifetime of integrated circuits formed from the wafer. Heretofore, however, charge carrier lifetime measurements were typically performed on test semiconductor wafers, not product semiconductor wafers. An obvious problem with performing charge carrier lifetime measurements on test wafers is that there may not be a sufficient correlation between the charge carrier lifetime measurement of a test wafer and the charge carrier lifetime of one or more corresponding product wafers.
It is, therefore, an object of the present invention to overcome the above problem and others by providing a method and apparatus for non-destructively determining a charge carrier lifetime of a product semiconductor wafer. Still other objects will occur to others upon reading and understanding the following detailed description.
The invention is a method of measuring a charge carrier lifetime of a semiconductor wafer that includes contacting an electrically conductive measurement probe to a surface of a semiconductor wafer to form a capacitor and applying a DC voltage having an AC voltage superimposed thereon between the measurement probe and the semiconductor wafer. The DC voltage is swept between a first voltage and a second voltage. The semiconductor wafer adjacent the contact between the measurement probe and surface of the semiconductor wafer is exposed to a light pulse. After the light pulse terminates, a change in capacitance of the capacitor over time is determined. From this thus determined change in capacitance, a charge carrier lifetime of the semiconductor wafer is determined.
The semiconductor wafer can include a dielectric overlaying semiconductor material. The measurement probe contacts the dielectric to form the capacitor whereupon the measurement probe defines a first plate of the capacitor, the semiconductor material defines a second plate of the capacitor and the dielectric defines an electrical insulator therebetween.
The measurement probe can include a dielectric that contacts a semiconductor wafer. The use of a measurement probe having dielectric enables formation of the capacitor when the probe is utilized to measure the charge carrier lifetime of a semiconductor wafer not having an overlaying dielectric.
At least the part of the measurement probe that contacts the semiconductor wafer can be formed from an elastically deformable material.
At the second voltage, the capacitor has a minimum capacitance value. In response to the light pulse, the capacitance value increases from the minimum capacitance value. After the light pulse terminates, the capacitance value decreases from the increased capacitance value to the minimum capacitance value.
The step of determining a change in capacitance can include determining a time rate of change in the capacitance of the capacitor. This time rate of change in the capacitance of the capacitor can be utilized to determine the charge carrier lifetime of the semiconductor wafer. The time rate of change in the capacitance of the capacitor is preferably determined temporally adjacent the termination of the light pulse. However, this is not to be construed as limiting the invention.
The exposure of the semiconductor wafer to the light pulse and the determination of the change in capacitance of the capacitor over time preferably occur in the presence of the second voltage. However, this is not to be construed as limiting the invention.
The invention is also a semiconductor wafer charge carrier lifetime measuring apparatus. The apparatus includes an electrically conductive wafer chuck for supporting a backside of a semiconductor wafer and an electrically conductive measurement probe. A movement means is provided for moving the measurement probe and a topside of the semiconductor wafer into contact when the wafer chuck is supporting the semiconductor wafer. The contact between the semiconductor wafer and the measurement probe forms a capacitor. An electrical stimulus means is provided for applying a DC voltage having an AC voltage superimposed thereon to the capacitor and for sweeping the DC voltage from a first voltage to a second voltage. A light source supplies a light pulse to the semiconductor wafer adjacent the contact thereof with the measurement probe. A measurement means is provided for measuring a change in capacitance of the capacitor over time after the light pulse terminates and for determining from the change in capacitance over time a charge carrier lifetime of the semiconductor wafer.
More specifically, the measuring means determines a time rate of change in the capacitance of the capacitor and determines the charge carrier lifetime of the semiconductor wafer from the time rate of change in the capacitance of the capacitor.
Lastly, the invention is a method of measuring a charge carrier lifetime of a semiconductor wafer that includes forming a capacitor with a top surface of a semiconductor wafer and sweeping a test voltage applied to the capacitor from a first voltage to a second voltage. A light pulse is applied to the semiconductor wafer whereupon the capacitance of the capacitor increases. A time rate of change in a decay of the capacitance of the capacitor is determined and a charge carrier lifetime of the semiconductor wafer is determined from the thus determined time rate of change in the decay of the capacitance of the capacitor.